SOM #4: Soil Organic Matter and its influence on biological processes
Soil health is described in many different ways, but one expression which is most useful is the definition from the USDA.
The USDA describes soil health as being “the capacity of a soil to function as a vital living ecosystem that sustains plants, animals and humans”. This view has been commended by leaders in the field of regenerative agriculture as a potential turning point in how important society and policymakers value soil. Including the word “living” in the words of some is a huge step forward, because it recognises that the soil is a “living entity” and not a benign material open to continued manipulation and exploitation without consequence.
In my previous blogs (click here to find them), we have been looking at the influences of soil texture on soil organic matter (SOM) content and how nutrient supply is affected by SOM. In this final blog of the series, NRM has been delving into the data to look at the differences in soil biological activity in different soil textures and how it might be influenced by SOM content.
Breakthroughs in soil testing
The soil is home to organisms of all shapes and sizes, making up 1-5% of total SOM levels. The majority of bacteria and fungi existing in soil (> 95%) were not able to be studied until recent times, but new molecular techniques are beginning to reveal the genetic fingerprints of previously unknown organisms.
Much of our current understanding of the roles of bacteria and fungi in soil is therefore derived from approaches which treat micro-organisms in soil as a single unit. This is referred to as the soil microbial biomass.
The most recognised and cost-effective method of evaluating soil biological activity is a test called Solvita®. The test was originally developed in the US several years ago, but it has been adapted to work in UK soils and it forms the biological basis of NRM’s soil health package.
What is the Solvita® CO2-Burst test and how is it useful for growers?
The Solvita® respiration CO2-Burst test is a proxy measure for the size of the microbial population in a soil. A carbon dioxide (CO2) measurement can be made when a dried soil is rewetted. It is the action of rewetting the soil sample that initiates this burst of CO2 from the microbial population living within it.
The majority of carbon dioxide released from soils is a result of living microbial populations going about their business, and a greater amount of CO2 released from soil suggests a greater microbial activity. The quantity of CO2 released is compared to a Solvita® Digital Colour Reader scale and is reported to be proportional to the microbial biomass living in the soil.
Most bacteria present in the soil obtain their energy and carbon from organic matter. They account for the general breakdown of organic matter and are involved in almost all organic processes, including a range of oxidation and reduction reactions that help to form healthy soil
How soil types affect SOM
The data in the following chart describes the proportion of samples measured in a series of CO2 burst categories for clay, clay loam, and sandy loam soils. The data indicates a shift to the right (greater respiration rate) for heavy (clay) and medium (clay loam) textured soils compared to lighter sandy loams. Both clay and clay loam soils had 20% more samples in the >150 CO2 burst categories compared to sandy loam. This higher microbial activity will be related to the fact that they also contained a greater SOM content.
The sandy loam soils seemed to peak at the 75-100 mg/kg CO2 burst category, and the number of samples declined as the CO2 burst category increased. More of the lighter textured soils contained less organic matter, and this might partly explain the lower biological activity.
How SOM affects different soil textures
The chart below describes the average CO2 burst measured in three soil textures within a range of SOM concentrations: 0 to 5%, >5 to 10%, and between >10 and 15%. When a soil texture contains more SOM, the average rate of CO2 respiration increases, and this is an indication of a larger microbial population.
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What does this mean for growers?
A soil with a higher organic matter content can support more diverse microbial life and activity and we know this leads to a more resilient system that is better functioning, delivering higher productivity and enhanced ecosystem services. Over the last few years, AHDB has published a series of soil health guidelines and indicators. One of the biological indicators highlighted is CO2 burst. The Soil Health project concluded that soils under arable management have good biological activity at >135 mg/kg CO2 burst and under grassland management at >180 mg/kg CO2 burst. If we apply these benchmarks to the NRM dataset, and assume the fields were under either arable or grassland management, on average only clay and clay loam soils containing between 10 and 15% SOM would be reaching that benchmark if managed in an arable rotation.
If we continue to apply this theory to sandy loam soils with the highest SOM content, on average under arable management, they will have moderate biological activity, and under grass it would reduce to low activity. Both clay and clay loam soils under arable management between 5 and 10% SOM meet the moderate activity benchmark, but that reduces to low activity when the SOM content drops below 5%. For soils under grassland management, CO2 bursts of less than 130 mg/kg are categorised as low activity, and this applies to all soil textures where the SOM is less than 10%.
A high organic matter content of soil isn’t a silver bullet, meaning that it can’t mitigate all other poor soil management practices that are inflicted upon it. A soil which contains a good level of organic matter can still be compacted or poached badly by animals, and when this happens soil biological activity is also affected. So, it’s worth bearing in mind that other management activities can negatively impact the biological communities you are trying to preserve. A well thought through soil management plan can highlight practices to be avoided and practices to engage at the right time. It is useful to note that mineral soils with very high SOM content under grassland management can indicate a layer of undecomposed litter at the soil surface. This suggests a lower pH and poor drainage, two conditions which reduce soil biological activity.
The chart below describes the NRM dataset in a different way by comparing the average CO2 respiration rate to the SOM content. The data suggests a strong relationship exists between those parameters and within light, medium and heavy soil types.
The Statistic R2 describes the influence of one parameter over another. In this analysis, the change in SOM content describes nearly 90% of the change in microbial activity. Overall, increasing SOM supports a greater abundance, diversity, and microbial communities that can exist in soils.
It is clear from the NRM dataset that SOM content is incredibly important for the health of your soil, and there seems to be no aspect of soil function that organic matter doesn’t influence and generally improve. We think it’s important to back up guidance and advice around soil organic matter with evidence, and benchmarking your farm is a good place to start.
How NRM can help
Measuring soil biology, organic matter and soil carbon couldn’t be easier. NRM’s soil health and CarbonCheck services include SOM analysis to help you qualify for the SFI, improve soil health, and get a head start on your carbon capture journey.
Click here to contact us directly or speak to your adviser for further information.
NRM is exhibiting at Groundswell on 26th & 27th June. Visit us at stand PFG9 to learn more about how we can help you optimise crop production.
References
Nicole Masters - For the Love of Soil (Strategies to Regenerate our Food Production System).
Institute of Organic Training & Advice: Research Review: Laboratory mineral soil analysis and soil mineral management in organic farming. (This Review was undertaken by IOTA under the PACA Res project OFO347, funded by Defra)